White LEDs are known in the art, and they are relatively recent innovations. It was not until LEDs emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination source based on an LED. Economically, white LEDs have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulb in lifetime, robustness, and efficiency. For example, white light illumination sources based on LEDs are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LEDs have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications.
There are several general approaches to making a white light illumination system based on light emitting phosphors. To date, most white LED commercial products are based on the approach shown in FIG. 1A, where light from a radiation source contributes directly to the color output of the white light illumination (in addition to providing excitation energy to a phosphor). Referring to the system 10 of FIG. 1A, a radiation source 11 (which may be an LED) emits light 12, 15 in the visible portion of the electromagnetic spectrum. Light 12 and 15 is the same light, but is shown as two separate beams for illustrative purposes. A portion of the light emitted from radiation source 11, light 12, excites a phosphor 13, which is a photoluminescent material capable of emitting light 14 after absorbing energy from the radiation source 11. The light 14 can be a substantially monochromatic color in the yellow region of the spectrum, or it can be a combination of green and red, green and yellow, or yellow and red, etc. Radiation source 11 also emits blue light in the visible that is not absorbed by the phosphor 13; this is the visible blue light 15 shown in FIG. 1A. The visible blue light 15 mixes with the yellow light 14 to provide the desired white illumination 16 shown in the figure.
Alternatively, a newer approach to making a white light illumination system has been to use non-visible radiation sources that emit light in the ultra-violet (UV) portion of the spectrum. This concept is illustrated generally at 20 in FIG. 1B, which illustrates an illumination system comprising a radiation source that emits in the non-visible such that the light coming from the radiation source does not contribute substantially to the light produced by the illumination system. Referring to FIG. 1B substantially non-visible light is emitted from radiation source 21 as light 22, 23. Light 22 has the same characteristics as light 23, but the two different reference numerals have been used to illustrate the following point: light 22 may be used to excite a phosphor, such as phosphor 24 or 25 to make photoluminescent light 26, 27, respectively, but the light 23 emitted from the radiation source 21 which does not impinge on a phosphor does not contribute to the white light output 28 from the phosphor(s) because light 23 is substantially invisible to the human eye.
Red-green-blue (RBG) backlighting systems are known in the art, as illustrated schematically in FIG. 2A. These conventional systems use a separate LED chip for each of the three colors needed for the backlighting system: red, green, and blue. The conventional RBG system of FIG. 2A employs a red LED 20 for providing red light 20L, a green LED 21 for providing green light 21L, and a blue LED 22 for providing blue light 22L. The disadvantages of such a system are that each LED requires an electrical current controller, so that three current controllers are needed for the system of FIG. 2A.
What is needed in the art is an improved green phosphor for use in white light illumination systems, single-color green illumination systems and illumination systems based on the present green phosphor, which may comprise a combination of a blue LED and/or UV LED chip; combinations of the present green phosphor with UV chips and with blue emitting chips; improved green phosphors in a plasma display panel, and the novel green phosphors in RGB backlighting systems, such that the number of current controllers controlling current to the LED chips is reduced.